ED is an abbreviation
for extra low dispersion, which describes a property of manufactured glass that is used to make lenses for cameras and telescopes
Extra low dispersion (ED) glass prevents chromatic aberration
due to the fact that it concentrates and directs the wavelength of light more effectively onto the focus point of the light path.
Lenses made from extra low dispersion glass are noted to have fewer air bubbles and deformities of their glass lenses which means they are less likely to cause image distortion in the resulting image.